Wireless Digital Communication System PA0 1) low PSD for a fixed E results in "signal hiding" PA0 2) wide bandwidth W (W&gt;&gt;R) reduces the effect of frequency selective fading PA0 3) the advantage of diversity combining due to a higher transmission (chip) rate PA0 4) data security by PN code sequence scrambling
FIG. 1 is a block diagram of a digital wireless communication system. An information sequence b[n], which could be, for example, speech, a data file, video, etc., is first processed in the transmitter unit 11 and converted to radio signals which are radiated via an antenna 15 over a wireless medium or channel 19. The channel 19 is the free space medium through which the radiated radio signals traverse before reaching the antenna 17 of a receiver unit 13. The receiver 13 performs the inverse operation of the transmitter 11, that is, it converts the detected radio signals into an information sequence intelligible to a user at the receiving end.
FIG. 2 is a block diagram of the transmitter 11 of FIG. 1. An information source 21 provides a binary sequence b[n], for example [01001100. . . ], where each 0 or 1 represents one bit of information. The sequence may originate, for example, from a digitized audio/video signal or a data file. R.sub.b, the rate at which bits are generated by the information source 21, is defined as the information data rate, or simply, raw bit rate and is measured in bits per second, or bps. The raw bit interval T.sub.b, measured in seconds, is the reciprocal of R.sub.b, i.e., T.sub.b =1/R.sub.b.
To combat noise and other disturbances of a wireless channel, a channel encoder 25 adds redundancy to the information sequence b[n]. The channel encoder 25 outputs a sequence c[n] comprising channel coded data bits, or simply coded data bits.
The code rate r of the channel encoder is equal to k/m, where m is the number of coded data bits corresponding to k information bits during one unit interval. Thus, R, the rate of the channel encoder's output stream c[n], is R.sub.b /r. Since, due to redundancy added by the channel encoder, m&gt;k, the coded data bit rate R is always larger than the information rate R.sub.b by a factor of 1/r. The coded data bit interval T is the reciprocal of R, i.e. T=1/R.
The baseband filter 29 shapes each coded data bit of the sequence c[n] into an analog waveform g(t) with a particular pulse shape, for example, a half-cycle of a sinusoidal waveform with amplitude of either +1 or -1, depending on the value of the coded data bits, or equivalently, with a phase of either 0 or .pi..
The final stage of a transmitter is the carrier modulator 33, or up-converter, which modulates g(t) with a sinusoidal carrier signal A.sub.o cos(2.pi.f.sub.o t) where A.sub.o is the amplitude and f.sub.o is the RF carrier frequency. In cellular systems, f.sub.o is typically in the range of 900 MHz and 1900 MHz. The resulting transmitted signal s(t) is EQU s(t)=g(t)A.sub.o cos (2.pi.f.sub.o t)
This type of modulation, where each binary digit of an input sequence is mapped to one of two carrier phases, is known as binary phase-shift keying (BPSK).
FIG. 3A shows the amplitude spectrum .vertline.S(f).vertline. of BPSK-modulated signal s(t) for a coded sequence in which data bits are alternating 1s and 0s, i.e., c[n]=[ . . . 101010 . . . ] at rate R bps. The transmission bandwidth 61A of s(t) is approximately equal to R, the coded data bit rate.
A similar but more effective modulation system, known as quadri-phase-shift keying (QPSK), maps every two binary digits of the input sequence to one of four carrier phases, e.g. .theta.=.pi./4, 3.pi./4, -3.pi./4, -.pi./4.
FIG. 3B shows the amplitude spectrum .vertline.S(f).vertline. of QPSK-modulated signal s(t) corresponding to the same alternating sequence as in FIG. 3A. The transmission bandwidth 61B is now R/2, half the bandwidth used by the BPSK system of FIG. 3A. Thus, by using QPSK modulation instead of BPSK, the binary data b[n] can be transmitted at a rate of 2R.sub.b bps in a given bandwidth since each branch (in-phase or quadrature) modulates its data at a rate of R.sub.b bps. In actual practice, the coded bit rate is less than 2B bps.
Variations of the QPSK modulation scheme have been adopted in current U.S. TDMA (time-division multiple access) cellular/PCS (personal communications services) Interim-Standards IS-54/136 and in the European GSM (Global System for Mobile communications) standard.
Direct Sequence Spread Spectrum
Direct-sequence spread spectrum (DS-SS) is a special type of modulation scheme in which a binary data sequence is directly modulated by a spectrum spreading sequence s[n] before carrier modulation. Note that we represent binary sequences with square brackets [ ], while analog waveforms are represented with their amplitudes expressed inside curly brackets { }. Note also that modulation, i.e., multiplication, of an analog waveform c(t) with amplitude { . . . , 1, 1, -1, 1, . . . } by another waveform x(t) with amplitude { . . . , -1, 1, -1, 1, . . .} is functionally equivalent to exclusive-OR (XOR) gating a binary sequence c[n]=[ . . . 0010 . . .] with another sequence x[n]=[ . . . 1010 . . . ] when waveform amplitudes {1, -1} are mapped into binary logic [0,1]. If waveforms and their corresponding binary sequences have the same clock rate, then the resulting analog waveform v(t) and binary sequence v[n] are equivalent, i.e., EQU c(t).times.x(t)=v(t)={ . . . , 1, 1, -1, 1, . . . }.times.{ . . . , 1, 1, -1, 1, . . . }={ . . . , -1, 1, 1, 1, . . . }
is equivalent to EQU c[n].sym.x[n]=v[n]=[ . . . 0010 . . . ].sym.[ . . . 1010 . . . ]=[ . . . 1000 . . . ]
where the symbol .sym. denotes the XOR operator.
FIG. 4 is a block diagram of a BPSK-modulated DS-SS transmitter. Here, the components correspond to those of FIG. 2, with the exception that an XOR gate 90 has been inserted between the channel encoder 25 and the baseband filter 29 to mix in a spectrum spreading sequence x[n]. The output of XOR gate 90 is a spread spectrum sequence v[n]. Each bit of the spectrum spreading sequence x[n] is defined as a chip. The chip rate, is denoted by R.sub.c. In general, the chip rate R.sub.c, measured in chips per second, is much greater then R, i.e., R.sub.c &gt;&gt;R. The chip interval T.sub.c is defined as 1/R.sub.c. The ratio of transmission bandwidth W to the coded data bit rate R is the spreading gain, SG which here is equal to W/R.
For the non-spread digital communication system of FIG. 2, R.congruent.W and therefore SG.congruent.1. For the DS-SS signal of FIG. 4, W&gt;&gt;R and, as a result, SG&gt;&gt;1. Since W.congruent.R.sub.c, spreading gain can also be defined as SG=R.sub.c /R=T/T.sub.c. For ease of implementation in practical systems, the ratio T/T.sub.c is usually an integer and is denoted by N.
FIG. 5A shows the amplitude spectrum .vertline.S(f).vertline. of s(t) for the case where a coded bit sequence c[n] of all 1s, i.e., [ . . . 1111 . . . ], is modulated, or equivalently, XORed, at XOR gate 90 with an alternating spreading sequence x[n]=[ . . . 0101 . . . ]. The spread code sequence v[n]=c[n].sym.x[n] is [ . . . 1010 . . . ].
The transmission bandwidth W 101A is equal to R.sub.c the chip rate, which is much greater than the coded bit rate R. There is no spectrum spreading because the alternating sequence has poor spectrum spreading characteristics.
As FIG. 5B shows, in general, a random sequence is an ideal spectrum spreading code. The power spectral density (PSD), or equivalently, the amplitude spectrum .vertline.S(f).vertline., of s(t) is relatively flat within the transmission bandwidth W 101B. Since the same spreading code must be generated at the receiving end for despreading, a very long, i.e., much larger than the spreading gain, pseudo-random noise-like periodic code sequence, or PN sequence, is used.
In the case of quadrature spectrum spreading and QPSK modulation, two spreading sequences s.sub.1 [n] and s.sub.2 [n] are used for the in-phase and quadrature-phase branches respectively.
The U.S. CDMA (code-division multiple access) cellular/PCS standard IS-95 specifies a technique using BPSK modulation with quadrature spreading in which two spreading sequences s.sub.3 [n] and s.sub.4 [n] and BPSK modulation with in-phase and quadrature branches are used. This scheme is known as BPSK with quadrature spreading.
Received signal-to-noise power ratio is defined as SNR=P/(N.sub.o W) where P is the received power level (in watts) of a transmitted signal s(t), and N.sub.o W is the total noise power in bandwidth W. N.sub.o is the (Gaussian) noise power spectral density. Since P=E/T=ER, where E the energy per coded data bit, EQU SNR=(E/N.sub.o) (R/W)=(E/N.sub.o) (1/SG).
Thus, EQU PSD=P/W=E (R/W)=E/SG.
The performance of a digital communication system is measured in terms of the error rate in decoded data bits at the receiver. This error rate is inversely proportional to the (coded) bit energy E. Transmitting a coded data sequence c[n] with a spread spectrum is preferred over non-spreading for the following reasons:
Direct-Sequence Code-Division Multiple Access (DS-CDMA)
In a DS-CDMA scheme, a multiple number of wireless users share a common bandwidth of W Hz with the same carrier frequency f.sub.o. The coded data bits c.sub.i [n] of user i are DS-SS modulated with a distinct spreading code x.sub.i [n] assigned to that user i. A DS-CDMA forward link (cell-site) transmitter supporting K users essentially comprises K parallel DS-SS transmitter units. The DS-CDMA receiver for user i is identical to a DS-SS BPSK receiver. Since multiple users share the same bandwidth W, each user's receiver must tolerate interference from other users. That is, s.sub.i [n].sym.s.sub.j [n], where i.noteq.j, is not an all-zero sequence. Consequently, a sequence received and decoded by one user will contain errors due to multiple access interference (MAI) from other users. In a cellular environment, multiple access interference is due not only to users from the same cell, but also to users from neighboring cells.
FIG. 6 shows a cell-site transmitter based on the U.S. CDMA IS-95 standard. In this case, the spectrum spreading code x.sub.ij [n] of user i in cell j is a combination of two codes: a cell-specific code PN p.sub.j [n], and a user-specific code w.sub.i [n]. Note that x.sub.ij [n]=p.sub.j [n].sym.w.sub.i [n]. Again, the cell-specific code sequence p.sub.j [n] is a very long (&gt;&gt;N) PN sequence and is generated at rate R.sub.p =R.sub.c. The user-specific codes w.sub.i [n] are preferably orthogonal Walsh codes which are generated at rate R.sub.w =R.sub.c. The repetition period N of a Walsh code is preferably equal to the spreading gain SG. For this reason, p.sub.j [n] is called a `long` code, and w.sub.i [n] is the `short` code. Since w.sub.i [n] is user-specific, and each user is assigned a channel, w.sub.i [n] is also known as the channelization code. The PN codes, p.sub.j [n] used primarily for spectrum spreading, are also called scrambling codes. Signals for the various users are combined by combiner 96 and are then filtered by the baseband filter 95 and modulated with the carrier at mixer 33.
An N-dimensional Walsh code is one of the N row vectors of an N-by-N Hadamard matrix. A 2-by-2 Hadamard matrix W.sub.2 is shown below: ##EQU1##
The two 2-dimensional Walsh codes corresponding to the two rows of the matrix are w(2,1)=[1 1] and w(2,2)=[1 0], where w(N,j) designates a Walsh code of dimention N from the j'th row of the NxN Hadamard matrix W.sub.N. All Walsh codes derived from a Hadamard matrix are mutually orthogonal. The dimension, also known as the code length, is equal to its repetition period N.
Higher-dimensional Hadamard metrices can be generated recursively from a lower dimention al Hadamard matrix In general, ##EQU2## where an asterisk (*) denotes the complement.
Walsh codes of dimensions 4 and 8 are illustrated below. ##EQU3##
For convenience, we refer to the IS-95 standard in which all users have a common information rate R.sub.b, as standard CDMA (STD-CDMA).
FIG. 7 is a block diagram of a STD-CDMA user transmitter. Consider, for simplicity, a STD-CDMA system with a spreading gain SG=4. Each coded bit from a sequence c.sub.i [n] from user i is mapped by the XOR gate 94 to a 4-dimensional Walsh code w(4,i) at a rate R.sub.w =R.sub.c. For example, for user 3, the sequence c[n]=[ . . . 1011 . . . ] is mapped to the sequence EQU d[n]=[ . . . w(4,3), w.sup.* (4,3), w(4,3), w(4,3) . . .] =[ . . . 1100 0011 1100 1100 . . . ]
The Walsh-coded sequence d[n] is next modulated with a PN sequence p[n] at XOR gate 92 at a rate R.sub.p =R.sub.c. Note that the spreading gain SG is R.sub.w /R=4, the chip rate R.sub.c is R.sub.p =R.sub.w, the transmission bandwidth W is R.sub.c, and the power density PSD is E/SG=E/4.
Now, consider again the despreading operation of user i in cell j. If an interfering user k is from the same cell as user i, e.g., cell j, then, EQU s.sub.ij [n].sym.s.sub.kj [n]=(p.sub.j [n].sym.w.sub.i [n]).sym.(p.sub.j [n]).sym.w.sub.k [n]=w.sub.i [n].sym.w.sub.k [n]
Because Walsh codes are orthogonal, the value of (w.sub.i [n].sym.w.sub.k [n]) over a period of N chips or samples is zero.
If, on the other hand, the interfering user k is also from a different cell from user i, e.g., cell m, then, EQU s.sub.ij [n].sym.s.sub.km [n]=(p.sub.j [n].sym.w.sub.i [n]).sym.p.sub.m [n].sym.W.sub.k [n])=p.sub.j [n].sym.p.sub.m [n]
which, over a period of N, is not equal to zero. In other words, there is a certain level of cross-correlation between two scrambling codes. Selection of PN long codes p.sub.j [n] with low cross-correlation property is crucial in reducing multiple access interference. PN codes with low autocorrelation, i.e., for non-zero time shifts, are also preferable for easy code acquisition at the receiving end.
According to the IS-95 standard, each user in a forward (base-to-mobile) link is assigned a 64-dimensional Walsh code and all users from the same cell are assigned a common cell-specific PN having a length of 32767.
Two fundamental limitations of the IS-95 standard are that every channel supports only a single low information rate of R.sub.b bps (9.6 kbps), and that the system is optimized for voice communication. Several new methods based on CDMA have been suggested to support higher information rates for non-voice communications such as Web surfing, data file transfer and other multimedia applications.
The multi-code CDMA (MC-CDMA) scheme disclosed in U.S. Pat. No. 5,442,625 to Gitlin et al provides higher user data rates by implementing a parallel combination of IS-95 CDMA traffic channels. For example, a user with a coded data rate of 2R is assigned two STD-CDMA channels. This requires multiple baseband transmitter/receiver units per user. The spreading gain SG is R.sub.w /R.sub.1 =R.sub.w /R.sub.2 =4, the chip rate R.sub.c is R.sub.p =R.sub.w, the transmission bandwidth W is R.sub.c and the power spectral density PSD is 2E/SG=E/2. The modulation scheme is no longer BPSK, because the code sequence can take on any of three values: 0, 1 and 2.
Variable spreading gain CMDA, or VSG-CDMA, disclosed in U.S. Pat. No. 5,751,761 to Gilhousen, provides higher data rates by using a lower dimensional Walsh code for Walsh mapping. For example, a user with the coded data rate 2R is assigned a Walsh code with dimension N/2. Where N=4, each bit of the sequence c[n] =[ . . . 1011 . . . ] is mapped to a 2-dimensional Walsh code, resulting in the sequence EQU d[n]=[ . . . 10 01 10 10 . . . ].
The Walsh-coded sequence d[n] is then modulated with a PN code p[n]. Now, the spreading gain SG is R.sub.w /2R=2, the chip rate R.sub.c is R.sub.p =R.sub.w, the bandwidth W is R.sub.c and the power spectral density PSD is E/SG=E/2.
The assignment of a parent Walsh code w(2,2)[n] to a VSG-CDMA user prohibits the assignments of its derivative codes w(4,2) and w(4,4) to two STD-CDMA users. F. Adachi et al., "Tree-structured generation of orthogonal spreading codes with different lengths for forward link of DS-CDMA mobile radio," Electronic Letters, January 1997, incorporated herein by reference, have shown a simple way of setting up a modified Walsh code structure which identifies both root and parent codes of a Walsh code using a tree-structured approach to modified Walsh code generation.
For a fixed E (energy per bit), both MC-CDMA and VSG-CDMA users require twice as much energy as a STD-CDMA user. In MC-CDMA, the increase in rate is due to code aggregation of STD-CDMA Walsh codes. In VSG-CDMA, the increase in rate is due to code blocking by using a lower-dimensional (parent) Walsh code. Compared to STD-CDMA users, both MC-CDMA and VSG-CDMA users with higher transmit power, or equivalently, larger PSD, are not power transparent.